Meanwhile, Rafael Nadal is into the last eight after defeating Russia Mikhail Youzhny 6-2 6-3.

The World No.5, who lost out in the third round last year, has won four titles since his return to action in February following a seven-month injury layoff.

Nadal, who is hoping to win an eighth French Open crown in Paris, told Spanish television that he was happy with his current level of fitness.

"We are just doing what we can every day," the 26-year-old told La Sexta.

Photos:Brand Max: Maria Sharapova

Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Maria Sharapova has capitalized on her on-court success by becoming one of tennis' most successful brands.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Her partnership with longtime agent Max Eisenbud has made her the highest-paid female athlete on the planet. They met 15 years ago when Sharapova was training at the IMG Academy in Florida. "There was so much going on and so many kids, so many parents to take care of, so it was actually fun to see him juggle all those different things and maintain a very clear vision of what he wanted to do," Sharapova recalls.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Pictured here during a shoot for CNN's tennis show Open Court, they are already plotting her future after tennis -- but she plans to play for several more years yet.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Sharapova with two of the most important men in her life -- hugging her father Yuri after winning her first grand slam title at Wimbledon as a 17-year-old in 2004, while Eisenbud looks on.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – That success allowed Eisenbud to court major sponsors, and helped Sharapova launch her own perfume among other non-tennis sidelines.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Sharapova has long been associated with Nike, and signed a reported eight-year deal with the sportswear giant in 2010 that could be worth up to $70 million.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Part of that sum includes royalties from her fashion collection with Nike subsidiary Cole Haan. Sharapova is pictured here at a promotional event in Tokyo in 2009.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Sharapova's "Sugarpova" candy collection is her first independent venture. "Everyone loves a treat and everyone loves candy. When I was young and I would finish a practice, what would I ask for? I would ask for little lollipops," she told Open Court.

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Photos:Brand Max: Maria Sharapova

Brand Max: Maria Sharapova – Her portfolio of endorsements keeps growing -- in April 2013 she was named as a brand ambassador for German car manufacturer Porsche. She has now won the tournament it sponsors two years in a row.

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Photos:Wimbledon to put a lid on it

Photos:Wimbledon to put a lid on it

Flying high – The All England Club, which organizes the Wimbledon Championship, has revealed that it will build a new roof on No.1 Court.

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Photos:Wimbledon to put a lid on it

Centre of attention – The iconic Centre Court has had a retracting roof in place since 2009, which has allowed players to continue their matches during rain showers.

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Photos:Wimbledon to put a lid on it

Taking cover – The $152 million project will see a roof put on No.1 Court, which will take the number of covered seats at the venue to 26,500.

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Photos:Wimbledon to put a lid on it

Finished product – This is an artist's impression of the completed project, which is expected to be ready in time for the 2019 tournament.

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Photos:Pride of Poland: The sisters Radwanska

Photos:Pride of Poland: The sisters Radwanska

Polish pride – The Radwanska sisters are putting Poland on the tennis map like never before.

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Photos:Pride of Poland: The sisters Radwanska

Trophy hunter – Agnieszka, 24, won the last of her 12 WTA titles in Australia earlier this year, at January's Sydney International.

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Photos:Pride of Poland: The sisters Radwanska

Rising up – Urszula, 22, was seeded at a grand slam for the first time this year, but the No. 31 exited the Australian Open in the first round at the hands of American Jamie Hampton.

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Photos:Pride of Poland: The sisters Radwanska

In the family – A young Urszula (left) and Agnieszka pose at home with their father Robert, a veteran tennis coach who has been their mentor.

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Photos:Pride of Poland: The sisters Radwanska

Break point – The sisters were surrounded by tennis paraphernalia from a young age.

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Photos:Pride of Poland: The sisters Radwanska

Net gain – Both learned tennis from an early age, with Agnieszka and Urszula starting at 5 and 4 respectively.

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Photos:Pride of Poland: The sisters Radwanska

Sisterly love – Urszula and Agnieszka say they have been best friends since childhood, with the pair still talking to one another every day where possible.

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Photos:Pride of Poland: The sisters Radwanska

On the way – The sisters' doubles partnership goes back to childhood. In 2012, they proudly represented Poland at the Olympic Games -- but suffered defeat in the second round.

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"I am in the quarterfinals and it makes me very happy to have another chance to play in Madrid in front of these magnificent fans.

"It's not the time to be talking more than necessary. I will try to enjoy every moment as much as possible."

Maria Sharapova booked her place in the quarterfinals with a comfortable 6-2 7-5 win over Germany's Sabine Lisicki.

And Sharapova, currently ranked second in the world, believes she's in great form going into the defense of her title at Roland Garros.

"I think when you win a Grand Slam on a surface, it would be pretty mean of me to say that I'm not comfortable on the surface," she told reporters.

"It took me many years to get to the level of where I am today. It certainly didn't happen overnight.

"I worked extremely hard on getting stronger and recovering better, moving better on the court, giving myself a better position on the court, especially after being on the defensive and trying to play aggressively as I always do on quicker surfaces."